why do expensive amplifiers produce a better soundstage

I use a 2003 Yamaha RX-Z9 RECEIVER in "Pure Direct" mode through my Merman speakers and have not heard anything that has ever "soundstaged" and "imaged" as well as my system in my room..., and I’ve been to CES and T.H.E. Show.  This is just the two speakers (18"/10"/AMT), and there is no eq or room correction, either electronic or physical being used. 

The system brings the venue into the room rather than taking the room to the venue. And the equipment has the volume and headroom to bring ANY SIZE venue into the room.  Total investment is <$6K.

https://youtu.be/OmWNOi6b4ak

It seems to me that if an amplifier contributes to soundstage and imaging, it is through improved channel separation and crosstalk rejection.

.,,,The Room‼️and speaker placement…. 

here is the answer to your question 

it comes down to parts quality an expensive amplifier uses higher quality better tolerance parts if you use cheap resistors they may not be of the same level they are reported to be same with capacitors enough variation in the circuit leads to loss of signal 

What is expensive?

$2k

$5K

$10K

$25K +++++++++++++++++++++++++++++++++

Those who are buying upper tier equipment also understand room acoustics to a degree. 

I agree that transducers have a greater impact than amplifiers-which have one job, to amplify the signal passed to it, but, of course, a quality designed and built component will allow the rest of the system to perform at its best. Price is not an exact indicator of quality, but unfortunately the old saying that you get what you pay for is generally, maybe not universally true in audio just like anything else.

With that said, the room has a great effect on soundstage and the spatial cues engineered into the recording dictate what is possible.